Syntax

Parameters

A pointer to an OVERLAPPED data structure that
contains the data used for asynchronous I/O.

If this parameter is NULL, all I/O requests for the hFile
parameter are canceled.

If this parameter is not NULL, only those specific I/O requests that were issued
for the file with the specified lpOverlapped overlapped structure are marked as
canceled, meaning that you can cancel one or more requests, while the
CancelIo function cancels all outstanding requests on a file
handle.

Return value

If the function succeeds, the return value is nonzero. The cancel operation for all pending I/O operations
issued by the calling process for the specified file handle was successfully requested. The application must not
free or reuse the OVERLAPPED structure associated with
the canceled I/O operations until they have completed. The thread can use the
GetOverlappedResult function to determine when
the I/O operations themselves have been completed.

If the function fails, the return value is 0 (zero). To get extended error information, call the
GetLastError function.

If this function cannot find a request to cancel, the return value is 0 (zero), and
GetLastError returns
ERROR_NOT_FOUND.

Remarks

The CancelIoEx function allows you to cancel
requests in threads other than the calling thread. The
CancelIo function only cancels requests in the same thread that
called the CancelIo function.
CancelIoEx cancels only outstanding I/O on the handle,
it does not change the state of the handle; this means that you cannot rely on the state of the handle because you
cannot know whether the operation was completed successfully or canceled.

If there are any pending I/O operations in progress for the specified file handle, the
CancelIoEx function marks them for cancellation. Most
types of operations can be canceled immediately; other operations can continue toward completion before they are
actually canceled and the caller is notified. The
CancelIoEx function does not wait for all canceled
operations to complete.

If the file handle is associated with a completion port, an I/O completion packet is not queued to the port if
a synchronous operation is successfully canceled. For asynchronous operations still pending, the cancel operation
will queue an I/O completion packet.

The operation being canceled is completed with one of three statuses; you must check the completion status to
determine the completion state. The three statuses are:

The operation completed normally. This can occur even if the operation was canceled, because the cancel
request might not have been submitted in time to cancel the operation.

The operation was canceled. The GetLastError
function returns ERROR_OPERATION_ABORTED.

The operation failed with another error. The
GetLastError function returns the relevant error
code.

In Windows 8 and Windows Server 2012, this function is supported by the following technologies.